Please Welcome to the Stage: Every Single Surprise Taylor Swift Guest

Who?! You might know Charli XCX from her pop megahit, "Boom Clap." Which, we'll admit, is still making appearances on our playlists.

Where?! Charli and Taylor shut it down in Toronto.

Why?! We actually haven't really seen these two hanging out before, but given Charli's current grasp on the pop music world, it would make sense that Taylor would welcome her to the stage.